chore(packaging/docker): Support multiple architectures (#762)

This finally adds support for multiple architectures!  

Images are now built using BuildKit (actually img), Dockerfiles were parameterized to allow this.
Containers are still built on alpine, using QEMU and binfmt as a kernel compatibility layer to allow RUN steps in these.

CircleCI has been fully revamped, to speed up builds and de-duplicate the configuration.
Makefile has been extended with additional logic to shadow the multi-arch to non-CI users.

Happy hacking on Raspberry Pi and friends :)